C5 - Work, Energy and Power Flashcards
How much work is done when holding up a heavy object in your hand?
None.
You exert a force on the book but there is no movement.
You do work when lifting the book vertically and a force is applied.
Energy is transferred when you do work. The book has gained GPE. If you let the book fall, it’s weight will do work which is transferred into kinetic energy.
What is work done equal to?
Energy transferred
What is work?
The product of force and the distance moved in the direction of the force.
Work = force * distance moved
It’s unit is joules.
How is work done calculated when the force is applied at an angle?
Work done = (F Cos O°) * x
W = FxCosO°
You need to know the angle between the force and direction of motion
What is perpetual motion?
The concept that, once the device has been started, it will continue to move and do work without any further input of energy.
This is impossible due to the principle of conservation of energy.
What is the principle of conservation of energy?
The total energy of a closed system remains constant - energy can never be created or destroyed; only transferred from one form to another
What is energy?
The capacity for doing work.
It is a scalar quantity with size not direction measured in joules
What is kinetic energy?
Energy due to the movement of an object
Calculated by 1/2 * m * v2
What is gravitational potential energy?
Energy due to the position of an object in the Earth’s gravitational field
Calculated by mgh
G is acceleration of freefall where it is assumed to be a constant of 9.81m/s.
It is gained will object to get higher and lost once object get lower
How are kinetic energy and gravitational potential energy related?
They are both energy so
mgh = 1/2 mv2
What is power?
The rate of work done
Measured in watts
How was power calculated?
In terms of work and energy
Power = work done / time
P = W / t
P = E / t
How are power, motion and force related?
There are times with a constant force has to be exerted on an object to maintain its constant speed.
E.g. The net force on a car travelling on a level road at a constant speed is zero. The rate of work done by the forward force (engine) is equal to the rate of work done against frictional forces acting on the car.
This enables power to be calculated.
P = W/t = Fx/t
(Speed of the car (v) is x/t therefore…)
P = Fv
What is efficiency and how is it calculated?
Is the percentage of how much energy is transferred into useful energy
Efficiency = (Useful output energy/total input energy) * 100
